At the height of my stocking process over the past year, I had 6 QT tanks running in the basement all at the same time. That was not really fun, but it wasn't like I was being forced in to it. I wanted to introduce a bunch of wrasses and anthias to the display tank at the same time, so I needed all of those QT tanks. I treated with prazipro for flukes, but watched and waited for signs of other diseases before treating with copper, or CP (failed ebay experiment), or antibiotics when problems showed up.
I ran into several cases of infection or infestation, but would estimate it at about 10% of the fish that I brought in, mostly from divers den. I lost two fish (out of 40+) over the course of the year to disease, but saved more than that by catching the issues in a small QT tank and then treating just the affected fish.
So it can be done with fish bought online, sight unseen (except for the pictures on divers den, for example), you just need to deal with vendors with good reputations (check out the search results for them on r2r before you buy), and go slow with adding them to your display tank. It also helps to pick fish that fit your tank and your target population (don't mix fish that won't get along, etc). Once you get the hang of it, reacting to sick fish in QT isn't as daunting as it first may seem. Better to buy healthy fish that you have seen being happy eaters with your own eyes from the start, but some of us have to take our chances with the internet.
And, the last thing I would add, use the QT period to get them feeding aggressively on foods that are high in nutrients like omega 3 acids. There are lots of references on r2r to the relationship of a natural diet and the fish's immune system and the role that plays is helping them thrive. Something as easy as high quality frozen food like LRS reef frenzy, or culturing live blackworms for the more adventurous, or mixing up raw seafood in your blender (for the truly bold and daring) works great.
